What determines how many species live in one place and how abundant they are? Many ecologists will reply: "The resources of the habitat and the strategies that the species use to exploit them." Ecologist Stephen P. Hubbell answers with a mathematical formula, which he described in a book last year titled, "The Unified Neutral Theory of Biodiversity and Biogeography." To his colleagues' consternation, the formula seems to work: for trees, bees, bats, birds, fish, frogs and others--accounting for observed patterns of diversity with just a few parameters, such as rates of birth and migration. You'd think ecologists would be ecstatic. But Hubbell started from a postulate that most consider preposterous: that one tree or one bird is just like any other. His patterns result solely from random fluctuations in births, deaths and the arrival of new species.

But isn't being different the way species dodge head-to-head competition and avoid driving each other extinct? Those differences could be incidental, the formula's success suggests. Yet in a study that Hubbell and colleagues published earlier in 2002 in Science magazine, the theory hit a bump in the road. Hubbell's previous tests had concerned biodiversity either at one locale or collectively within a large region--which ecologists call "alpha" and "gamma" diversity, respectively. This time the test was what ecologists term "beta" diversity: how species demographics vary across a landscape. A careful setting of the parameters enabled the formula to predict changes across short, intermediate or long distances (0 to 0.1 kilometers, 0.2 to 50 km or 50 to 1,400 km) in forests of Panama and the Amazon. But no setting accounted for beta diversity as a whole.

With the theory's first failure, the time seemed ripe to ask Hubbell about his radical assumptions. Scientific American phoned him at his office at the University of Georgia, Athens.

SA: In the beta-diversity study of earlier this year, the parameter you focused on was seed dispersal. Why is that?

SH: Well, we were taking data on the occurrence of tree species at varying distances. If trees couldn't disperse anywhere, seeds would just land under the mother and that tree would replace itself with its own babies. There'd be no probability of sampling the same tree twice in different places. But with dispersal, they [the trees] have a chance to spread elsewhere, and so what you can do with the theory is to estimate what rate of spread is necessary to have the observed probabilities of drawing two trees of the same species from two places at a distance, r, apart.

SA: But no single rate explained the probabilities you observed over all distances. That's a problem for your assumption that one set of parameters describes all trees, isn't it?

SH: It's only a problem for the simplest version of the theory. In other words, the one published in the book is highly simplified. Applied to trees, it [the simplified version of the theory] says that all seeds effectively move a fixed distance--the mean distance--and of course we know that isn't true. You could have a more realistic neutral theory, in which lots of the seeds fall right beneath the mother, a few go farther, and even fewer go farther than that. If every tree species followed the same distribution of dispersal distances, then it could still be neutral.